1. Field of the Invention
The invention relates to phase shifting networks with multiple outputs between which a constant phase difference (typically 90.degree.) is maintained over a large frequency range. The invention further relates to phase shifting networks having this capability which may be used at microwave and millimeter wave frequencies and which are suited to monolithic fabrication integrating both active and passive components.
2. Prior Art
Phase shifting networks are known with multiple outputs between which a constant phase difference is maintained over a range of frequencies. A class of networks termed "all pass networks" are useful in this connection. All pass networks maintain a constant amplitude transfer characteristic over a band of frequencies. When such all pass networks are used in pairs to achieve two outputs in quadrature, the quadrature relationship may often be maintained over a substantial range of frequencies.
Paired all pass networks using resistors and capacitors as the passive elements and vacuum tubes or transistors as the active elements have been used for this purpose for some time. The principal earliest application for such networks has been in the audio spectrum for transmitter modulation. At radio frequencies, usually above 40 MHZ, 3 db couplers combined with transmission line phase shifters have been used to produce two coherent signals of substantially constant amplitude having a constant phase difference of 90.degree. over a range of frequencies, also for use in the modulation process.
The demand for coherent quadrature phased signal sources exists at higher radio frequencies where it is desired to form or to steer beams in antenna arrays. The use is both in reception and in transmission and for both communications and ranging. In such applications, a processor, termed a "beam former", may be required to adjust the phase of a signal in the path of each element in the array. The processor in this case must provide the same phase shift to all components of a broadband signal in the associated element. Broadband signal processing over bandwidths as high as one gigahertz are of current interest. In addition, a demand exists for beam formers covering large segments of the microwave and millimeter wave spectrum.
In a computer controlled beam former, a phase splitter may represent the first component of a "vector generator" following the oscillator. The phase splitter is then followed by a multiple output phase shifter which operates upon the two outputs which are of equal amplitude but opposite phase to provide two or four outputs between which phase quadrature is maintained. The quadrature components are then coupled to a pair of digitally controlled signal scalers such as that described in U.S. Pat. No. 4,638,190. In the scaler, the two orthogonally related coherent signals are each digitally scaled in proportions designed to form a resultant vector having a desired phase angle when the scaled components are recombined. The resultant vector, formed when the scaled components are recombined, constitutes a phase shifted replica of the original input signal.
The demand for such phase shifters now exists from moderate radio frequencies to microwave and millimeter wave frequencies. At the higher frequencies a need exists for compact phase shifters in monolithic configurations in which both active and passive components may be integrated on a common high performance substrate. Since the processing of low level signals requires that the signal to noise performance must not be significantly degraded, the need has arisen for phase shifting networks that provide minimum reductions in the signal to noise ratio. Finally, increasing bandwidth requirements have created a demand that such phase shifters be capable of maintaining a constant phase difference over an extended range of frequencies.